Good luck with your treatment.
In my conversation (titled Minor Q - Morning or Afternoon) -- still on page 1 as of now, there was some discussion about Bladder fullness and control.
The standard instructions at my center are to drink 500 ml one hour before the session. When I did that for my planning CT session, my bladder was near "maximum size", and they suggested I let out some urine, and redid the planning scans with bladder at a more comfortable size. They suggested I'm well hydrated (and said most men are not well hydrated), so they suggested that for me, I should drink 500 ml 30 minutes before the session, and that has worked well.
* their suggestion to drink 2 l per day is to ensure you are well hydrated, so that when you drink before your session, your bladder fills up quickly (rather than be absorbed by the body). Not feeling the initial urge for 50-60 minutes may be because you are not well hydrated. So try to be well hydrated first.
* the staff can advise you how to adjust the routine (like they did for me, suggesting I drink only 30 minutes before).
* the session themselves take no more than 15 minutes, so you won't need to hold for 40 minutes. If you are too full right before your session, you can let some out to be more comfortable.
* the centers using full bladder protocol will scan you before every radiation is applied, and can make minor adjustments to account for different degrees of bladder fullness.